Protecting aluminum from corrosion



Patented Feb. 6, 1934 i I I PROTECTING ALUMINUM FROM CORROSION Junius D. Edwards, Oakmont, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application April 29, 1933, Serial No. 668,629,and in Canada June 20, 1931 10 Claims.

This invention relates to the covering of the surfaces of aluminum and aluminum base alloys, herein collectively designated as aluminum surfaces, with a coating capable of resisting the action of corrosive agencies. The invention particularly contemplates the provision on said aluminum surfaces of a corrosion resistant oxide coating.

By various known processes aluminum surfaces may be provided with what is generally termed an oxide coating; such a coating is composed in large part of aluminum oxide. The coatings may be produced by several methods, which methods, with few exceptions, comprise a reaction, chemical or electro-chemical, between the aluminum surface and a solution of a chemically active substance or substances. The present invention refers to coatings produced by these artificial means and which are adsorptive. It is a further desirable characteristic of such coatings that they be comparatively heavy or thick, and also resistant to abrasion, but a comparatively thin coating may be satisfactory especially if its adsorptive power is high. The very thin film of oxide naturally forming on aluminum surfaces is not herein referred to as an oxide coating.

These oxide coatings have one very definite purpose of protecting aluminum surfaces against the action of corrosive agencies. In the performance of this function the oxide coatings find an important commercial justification. It is an important object of this invention to provide methods whereby the protection against corrosion afforded by an oxide coating on an aluminum surface may be substantially. increased and given longer useful life. A further object of the invention is the provision of oxide coated aluminum objects which may be used successfully under severe corrosive conditions.

To these and other ends the invention comprises the production of corrosion resistant coatings by forming adsorbent oxide coatings on aluminum surfaces and thereafter impregnating said coatings by treating them with a solution of soluble salt of chromic acid herein designated chromate, in which term I include dichromate. The dichromates are usually preferred, as I have found that an oxide coating of given adsorptive power will adsorb about twice as much hexavalent chromium from a dichromate solution as from a chromate solution. The alkali metal chromates and dichromates are preferred, as they give excellent results and are available at low cost. The oxide coating is first formed on the aluminum surface. For instance the aluminum is made the anode of an electrolytic cell, the electrolyte of which is an aqueous solution of sulphuric acid.

External electrical energy is impressed on the cell with the result that a layer of oxide is formed on the surface of the aluminum anode. This oxide coating formed is adsorptive and may be of varying thickness. Or the aluminum may be immersed in hot solutions of sodium carbonate and alkali dichromate for a short period of time. with the resulting formation of an adsorptive oxide coating on the surfaces thereof. Other methods of forming these oxide coatings are well known, and may be used, but I prefer the methods outlined above.

The aluminum, having been provided with an adsorptive oxide coating, is preferably washed to remove traces of residual solution remaining from the coating process and is then ready for the second step of the process. The second step consists in treating the oxide coated aluminum surface with a solution containing soluble chromate. Simple immersion is the most feasible method of treatment, and in commercial practice the oxide coated aluminum is immersed in the solution of soluble chromate, but other methods of conveying the solution to the surface will give good results. As a result of these treatments the soluble chromate is adsorbed in and on the adsorptive oxide coating. The oxide coated aluminum is then, if desirable, rinsed and dried.

The final product of my process has exceptional properties. Its resistance to the action of corrosive agents, even of a severe character, is decidedly greater than the resistance of an oxide coated aluminum which has not, in accordance with my invention, been treated with a solution of soluble chromate. Despite its soluble nature the chromate adsorbed in the oxide'coating is strongly bound therein and is leached out very slowly by the action of water, as is evidenced by the fact that oxide coated aluminum surfaces which have been treated in solutions of soluble chromates, even when the solution is held at ordie nary temperatures, do not lose their highly resistive property in monthsof exposure to the action of spray from a 20 per cent salt solution. For instancea group of aluminum alloy specimens were oxide-coated electro-chemically in sulphuric acid solution and a part of the group were then treat- 105 ed with soluble chromate in accordance with my invention. Both the treated and untreated oxide coated specimens were exposed for 52 weeks to the action of a spray of 20 per cent salt solution.

Those specimens treated in accordance with my with the chromate solution is effected at temperatures above about 80 C. and up to and including the boiling temperature of the solution. As the result of such a treatment not only is the soluble chromate adsorbed in and on the oxide coating but the pores of the coating are apparently closed so that, under the action of weather, moisture and similar agents, the removal of any important amount of chromate is practically precluded. Moreover, treatment at these temperatures has the important advantage of causing the adsorption of 2 or 3 times as much chromate as is adsorbed at ordinary room temperatures. Because of this increased adsorption activity effective amounts of chromate can be adsorbed from solutions of very low concentration in relatively short periods of treatment. I

The following tests are illustrative of the improved results obtained by practicing my invention at the elevated temperatures aforesaid.

Samples of duralumin, an aluminum alloy susceptible to attack by corrosive agencies, were oxide coated by treatment with a hot sodium carbonatepotassium dichromate solution. A sampic was then treated, in accordance with one aspect of my invention, by boiling it for 30 minutes in a 5 per cent solution of potassium dichromate. Both samples were subjected for one year to a continuous spray of 20 per cent salt solution. The oxide coated but untreated specimen lost, in this period, 31 per cent of its elongation, whereas in the oxide coated and treated I specimen there was in this period no measurable loss.

In another test aluminum was coated by anodic oxidation in a solution of sulphuric acid. The oxide coated aluminum was then washed and immersed for 20 minutes in a boiling solution containing 0.5 per cent of sodium dichromate. The specimen was rinsed and dried and a precise chemical analysis showed that the coating had a chromate content, expressed as chromium, of 0.86 gram per square meter. The sample was then immersed in a beaker of water at ordinary room temperature and allowed to remain therein for a period of two months. Analysis of the chromate content of the water at the end of this period showed that only an amount equivalent to 0.04 gram of chromium per square meter had been lost out of the coating.

By a soluble chromate I mean, in this speci-- fication and the claims appended hereto, a chromate or a dichromate or mixtures of these compounds. Very small amounts of chromic acid may be present in the treating solution without serious detriment but there is apparently no advantage in the presence of the acid and when the solution is used at elevated temperatures serious difliculties may result. It is often found that an oxide coated aluminum surface loses its oxide coating completely when boiled in a chromic acid solution containing, for instance, 1 per cent of chromic acid. An oxide coating was decreased in thickness from 0.6 mil to 0.16 mil by boiling for 20 minutes in a solution containing 0.5 per cent potassium dichromate and 0.5 per cent chromic acid.

This application is a continuation in part of my application for United States Letters Patent, Serial No. 648,989, filed December 2'7, 1932; in part of my application for United States Letters Patent, Serial No. 613,789, filed May 26, 1932; and in part of my application for United States Letters Patent, Serial No. 412.744. am August 2,

I claim- 1. The method of producing corrosion resistant coatings on aluminum by forming on the aluminum surface an adsorbent oxide coating and thereafter treating said coating with a chromate solution.

2. In a method of treating aluminum articles to render the surface thereof resistant to prolonged exposure to corrosive influences, forming on the surface of the article an adsorbent oxide coating, and thereafter adsorbing in said coating, chromate from a solution thereof.

3. In a method of treating aluminum articles to render the surface thereof resistant to prolonged exposure to corrosive influences, forming on the surface of the article an adsorbent oxide coating, and thereafter adsorbing in said coating an alkali metal dichromate from a solution thereof.

4. In a method of treating aluminum articles to render the surface thereof resistant to prolonged exposure to corrosive influences, forming on the surface of the article by anodic oxidation in an electrolyte containing sulphuric acid an adsorbent oxide coating, and thereafter adsorbing in said coating, chromate from a solution thereof.

5. The method of producing corrosion resistant 1 5 coatings on aluminum by forming on the aluminum surface an adsorbent oxide coating and thereafter treating said coating with a chromate solution, the said solution being maintained during treatment at a temperature above about centigrade.

6. The method of producing corrosion resist: ant coatings on aluminum by forming on the aluminum surface an adsorbent oxide coating and thereafter treating said coating with a solution containing an alkali metal chromate, the said solution being maintained during'treatment at a temperature above about 80 centigrade.

'7. The method of producing corrosion resistant coatings on aluminum by forming on the aluminum surface an adsorbent oxide coating and thereafter treating said coating with a dichromate solution, the said solution being maintained during treatment at a temperature above about 80 centigrade.

8. The method of producing corrosion resistant coatings on aluminum by forming on the aluminum surface an adsorbent oxide coating and thereafter treating said coating with a solution containing alkali metal dichromate, the said solution being maintained during treatment at a temperature above about 80 centigrade.

9. As a new article of manufacture, an article of metallic aluminum rendered resistant to corrosion by an adsorbent oxide coating on its surface containing chromate adsorbed in said coating after formation of the same, as evidenced by the fact that the coating can be subjected to washing with water for more than an hour without losing the corrosion resistance imparted by the adsorption of chromate.

10. As a new article of manufacture, an article of metallic aluminum having its surface rendered resistant to exposure to corrosive influences by an adsorbent oxide coating on its surface, im- 145 pregnated with a chromate adsorbed therein.

JUNIUS D. EDWARDS. 

